The Electron Column

In a SEM equipment, the electrons which result from the emission from a filament located in the electron gun are accelerated, with the help of a voltage ranging from 1 to 30keV (see Figure 4.10) [8,52], The electron emission event takes place in a vacuum milieu ranging from 10-4 to 10-10 Torr. Then, the accelerated electrons are directed to the specimen by a series of electromagnetic lenses in the electron column (see Figure 4.10) [8,52],... 

Figure 7.9 is a diagrammatic cross-section of a scanning electron microscope. The components used to obtain a focused electron beam at the sample constitute the electron column. This must be maintained under a secondary vacuum. 

The electron column comprises four scries of elements ... 

The essential elements of an electron microprobe are the electron column and the wavelength dispersive spectrometers (Fig. 8.5). The electron column is very similar to that of a scanning electron microscope. Certain elements are added to it (such as a beam controller, viewfinder, etc.) to make it an instrument dedicated to elemental microanalysis. 

The section used to produce the electron probe is the electron column (Fig. 8.5). 

As WDS analysis is sequential, it is practical to use several spectrometers of this type around the electron column but, for obvious reasons of available space (the diameter of the Rowland circle is approximately 30 cm), we are limited to 2 spectrometers mounted obliquely or 5 vertical spectrometers. The second configuration is generally chosen since it can be used to conduct simultaneous analysis of S elements on the same point. 

Both the electron column and the adjacent spectrometer are highly evacuated, a circumstance that improves light element detectability. With a crystal spectrometer all elements down to boron (Z = 5) can be detected. 

There are three components to the focal spot size df that strongly influence the design of the electron column, namely, the geometric focus d of the crossover dco, the spherical aberration d, and the chromatic aberration d. The geometric f ocus of the crossover is controlled by the demagnification of the electron lens and is given by... 

Molecular point-group symmetry can often be used to determine whether a particular transition s dipole matrix element will vanish and, as a result, the electronic transition will be "forbidden" and thus predicted to have zero intensity. If the direct product of the symmetries of the initial and final electronic states /ei and /ef do not match the symmetry of the electric dipole operator (which has the symmetry of its x, y, and z components these symmetries can be read off the right most column of the character tables given in Appendix E), the matrix element will vanish. 

The positive column is a region in which atoms, electrons, and ions are all present together in similar numbers, and it is referred to as a plasma. Again, as with the corona discharge, in mass spectrometry, plasmas are usually operated in gases at or near atmospheric pressure. 

Positive ions formed near the positive electrode (anode) are repelled by it and move into the positive column. Electrons that reach proximity to the anode are accelerated somewhat because... 

If the electrodes are moved closer together, the positive column begins to shorten as it moves through the Faraday dark space because the ions and electrons within it have a shorter distance through which to diffuse. Near the cathode, however, the electric-field gradient becomes steeper and electrons from the cathode are accelerated more quickly. Thus atom excitation through collision with electrons occurs nearer and nearer to the cathode, and the cathode glow moves down toward the electrode. 

As described above, the mobile phase carrying mixture components along a gas chromatographic column is a gas, usually nitrogen or helium. This gas flows at or near atmospheric pressure at a rate generally about 0,5 to 3.0 ml/min and evenmally flows out of the end of the capillary column into the ion source of the mass spectrometer. The ion sources in GC/MS systems normally operate at about 10 mbar for electron ionization to about 10 mbar for chemical ionization. This large pressure... 

Good semiconductors are drawn from the central columns. Groups 13, 14, and 15 (111,IV, and V), of the Periodic Table, where the atoms tend to be nonpolar. Eor this reason, and because of the giant size of the wave functions, the electron-atom interaction is very weak. The electrons move as if in free space, colliding with the atomic lattice rather infrequendy. 

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